Communication system for inter-cell coordination

ABSTRACT

System and methods for intercell coordination are provided. The system includes a macro terminal and a macro base station that allocate a radio resource for a sounding signal and determine a pattern of a sounding signal, in advance. The macro base station provides a femto base station included in the system with information about the pattern of the sounding signal, and the information about the radio resource of the sounding signal. The femto base station may receive the sounding signal from the macro terminal based on the information, and thus, the femto base station may determine whether the macro terminal exists and may determine a channel between the macro terminal and the femto base station.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit under 35 U.S.C. §119(a) of KoreanPatent Application No. 10-2009-0101222, filed on Oct. 23, 2009, in theKorean Intellectual Property Office, the entire disclosure of which isincorporated herein by reference for all purposes.

BACKGROUND

1. Field

The following description relates to a hierarchical cell communicationsystem including a femto base station and a macro base station, and moreparticularly, to a system where a femto base station recognizes a macroterminal or recognizes a channel between the femto base station and themacro terminal in a hierarchical cell communication system.

2. Description of the Related Art

Recently, studies have been directed towards a hierarchical cellcommunication system including a femtocell, a macrocell, and one or moremobile terminals. In the hierarchical cell communication system,interference from the femtocell to the macrocell can decrease theefficiency of the overall system. For example, a femtocell may interferewith a signal transmitted from the macrocell to a mobile terminal ormacro terminal.

Various methods have been proposed to prevent inter-cell interference,such as coordinated beamforming and the like. However, these methodsdemand exchanging/sharing of channel information between cells inreal-time. The exchanging/sharing of the channel information betweencells in real time in the hierarchical cell communication systemincluding the femtocell may be difficult. The femtocell is not directlyconnected with the macrocell, the femtocell is connected with themacrocell via a network, and thus, information transmitted/receivedbetween the femtocell and the macrocell incurs a delay.

SUMMARY

According to one general aspect, there is provided a transmission powercontrol method of a base station, the method including receivingsounding signal information of a macro terminal from a macro basestation, detecting a sounding signal transmitted by the macro terminalbased on the received sounding signal information, estimating a channelbetween the macro terminal and the base station based on the detectedsounding signal, and processing information about the estimated channelto control a transmission power of the macro base station and the basestation.

The method may further include requesting the sounding signalinformation from the macro base station.

The method may further include transmitting, to the macro base station,an acknowledgment message indicating that the sounding signalinformation was received.

The detected sounding signal may indicate at least one of informationidentifying the macro terminal, information about a radio resource usedby the macro terminal, and information about a quality of service (QoS)of the macro terminal.

The sounding signal information of the macro terminal may include atleast one of information about a pattern of the sound signal,information about radio resource for the sounding signal, andinformation about a transmission period of the sounding signal.

The processing of the information about the estimated channel mayinclude transmitting information about the estimated channel to themacro base station to control the transmission power of the macro basestation and the base station.

The processing of the information about the estimated channel mayinclude determining the transmission power of the base station based onthe information about the estimated channel.

The method may further include receiving information about thecontrolled transmission power of the base station from the macro basestation.

According to another general aspect, there is provided a transmissionpower control method of a base station, the method including receivingsounding signal information of a macro terminal from a macro basestation, detecting a sounding signal transmitted by the macro terminal,based on the received sounding signal information of the macro terminal,and adjusting the transmission power of the base station based on thepower of the detected sounding signal.

The adjusting may include comparing the power of the detected soundingsignal with at least one comparison value, and increasing or decreasingthe transmission power based on a result of the comparison.

According to another general aspect, there is provided a transmissionpower control method of a base station, the method including receivingsounding signal information of a macro terminal from a macro basestation, detecting a sounding signal transmitted by the macro terminalbased on the received sounding signal information of the macro terminal,and

performing one of estimating a channel between the macro terminal thebase station based on the sound signal and generating a beamformingvector for a femto terminal based on the estimated channel, or receivinginformation about a channel between the macro terminal and the basestation from the macro terminal in response to the detection of thesounding signal, and generating the beamforming vector for the femtoterminal based on the received information about the channel.

The method may further include requesting the information about thechannel between the macro terminal and the base station from the macroterminal, in response to the detection of the sounding signal.

According to another general aspect, there is provided a transmissionpower control method of a macro terminal, the method including receivinga request, from a macro base station, to transmit a sounding signal, andtransmitting the sounding signal to a base station, wherein the soundingsignal indicates at least one of information identifying the macroterminal, information about radio resource for the macro terminal, andinformation about a QoS of the macro terminal, and sounding signalinformation is provided in advance to the base station from the macrobase station, the sounding signal information including at least one ofinformation about a pattern of the sounding signal, information aboutradio resource for the sounding signal, and information about atransmission period of the sounding signal.

According to another general aspect, there is provided acomputer-readable storage medium storing a program to controltransmission power of a base station, the computer-readable storagemedium comprising instructions to cause a computer to receive soundingsignal information of a macro terminal from a macro base station, detecta sounding signal transmitted by the macro terminal based on thesounding signal information of the macro terminal, estimate a channelbetween the macro terminal and the base station based on the detectedsounding signal, and process information about the estimated channel tocontrol the transmission power of the macro base station and the basestation.

According to another general aspect, there is provided acomputer-readable storage medium storing a program to controltransmission power of a macro terminal, the computer-readable storagemedium comprising instructions to cause a computer to receive a request,from a macro base station, to transmit a sounding signal, and transmitthe sounding signal to a base station, wherein the sounding signalindicates at least one of information identifying the macro terminal,information about radio resource for the macro terminal, and informationabout a QoS of the macro terminal, and sounding signal information isprovided in advance from the macro base station to the base station, thesounding signal information including at least one of information abouta pattern of the sounding signal, information about radio resource forthe sounding signal, and information about a transmission period of thesounding signal.

Other features and aspects may be apparent from the followingdescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

to FIG. 1 is a diagram illustrating a conventional example of amacrocell communication system based on a fractional frequency reuse(FFR) scheme.

FIG. 2 is a diagram illustrating another conventional example of ahierarchical cell communication system based on a FFR scheme.

FIG. 3 is a diagram illustrating a conventional example of ahierarchical cell communication system having difficulty in sharinginformation in real time.

FIG. 4 is a diagram illustrating a conventional example of ahierarchical cell communication system including a plurality of femtobase stations.

FIG. 5 is a diagram illustrating a process of an example hierarchicalcell communication system.

FIG. 6 is a diagram illustrating an example of a change in transmissionpower of a macro base station and a femto base station as a macroterminal moves.

FIG. 7 is a diagram illustrating a process of an example hierarchicalcell communication system that applies a long-term dynamic spectrummanagement scheme.

FIG. 8 is a flowchart illustrating an example of a method of a femtobase station in a hierarchical cell communication system that applies avoluntary dynamic spectrum management scheme.

FIG. 9 is a diagram illustrating an example of leakage-based beamforming

FIG. 10 is a flowchart illustrating an example of a method of femto basestation in a hierarchical cell communication system that appliesleakage-based beamforming.

FIG. 11 is a diagram illustrating a process of an example hierarchicalcell communication system that applies leakage-based beamforming.

FIG. 12 is a diagram illustrating another process of an examplehierarchical cell communication system that applies leakage-basedbeamforming.

Throughout the drawings and the detailed description, unless otherwisedescribed, the same drawing reference numerals should be understood torefer to the same elements, features, and structures. The relative sizeand depiction of these elements may be exaggerated for clarity,illustration, and convenience.

DETAILED DESCRIPTION

The following detailed description is provided to assist the reader ingaining a comprehensive understanding of the methods, apparatuses,and/or systems described herein. Accordingly, various changes,modifications, and equivalents of the systems, apparatuses and/ormethods described herein will be suggested to those of ordinary skill inthe art. The progression of processing steps and/or operations describedis an example; however, the sequence of steps and/or operations is notlimited to that set forth herein and may be changed as is known in theart, with the exception of steps and/or operations necessarily occurringin a certain order. Also, descriptions of well-known functions andconstructions may be omitted for increased clarity and conciseness.

A femtocell and a femto base station described in the presentspecification are examples of base stations that extend the networkcoverage of a service provider, for example, in a home, small business,and the like. Examples described herein may be applicable to acommunication system including various types of base stations, inaddition to the femtocell and the femto base station. The base stationmay include a relay and a radio remote head (RRH) in a heterogeneousnetwork. The base station may include a femto base station, a pico basestation, a home eNode B, and the like.

FIG. 1 illustrates a conventional example of a macrocell communicationsystem based on a fractional frequency reuse (FFR) scheme.

Referring to FIG. 1, the macrocell communication system including amacro base station that separately manages a frequency resource, F1+F2,for an inner cell and a frequency resource, F3, for an outer cell, basedon a FFR scheme. The frequency resource F1+F2 for the inner cell may bereused by an adjacent cell and the frequency resource F3 for the outercell is not used by an adjacent cell. The FFR scheme may increaseefficiency in using a radio resource by reusing the frequency resourceF1+F2 for the inner cell, and also may improve a capacity of a user inthe outer cell by allocating the frequency resource F3 for the outercell to prevent interference from the adjacent cell. That is, the macrobase station may allocate a higher transmission power in the frequencyresource F3 for the outer cell based on the FFR scheme as illustrated ina transmission power graph of FIG. 1

When the adjacent cell of the macrocell is another macrocell, the twomacrocells may share channel information in real time. However, when theadjacent cell is a femtocell, the macrocell and the femtocell may incurdifficulty in sharing channel information in real time, because thefemtocell is connected to the macrocell via the Internet. Also, alocation of the macrocell is fixed, whereas a location of the femtocellmay be freely changed. Accordingly, when the FFR scheme is used,interference from the femtocell to the macro terminal occurs.Accordingly, based on the FFR scheme, the hierarchical cellcommunication system including the macro cell and the femto cell willstruggle to share data in real time.

FIG. 2 illustrates another conventional example of a hierarchical cellcommunication system based on a FFR scheme.

Referring to the example shown in FIG. 2, the hierarchical cellcommunication system includes a macro base station, a macro terminal,and a femto base station. The hierarchical cell communication systemuses the FFR scheme, and thus, a frequency resource F1+F2 for an innercell and a frequency resource F3 for an outer cell are distinguishedfrom each other.

When the macro terminal and the femto base station are far from eachother, interference from the femto base station to the macro terminal issmall to none. However, as the macro terminal moves closer to the femtobase station, the interference increases. Also, interference may occurwhen a location of the femto base station is changed. Accordingly, theinterference from the femto base station to the macro terminal may notbe overcome with the FFR scheme.

In addition, to control interference that the femto base station maycause the macro terminal, the macro base station and the femto basestation need to share information about a channel between the femto basestation and the macro terminal. However, the femto base station is notdirectly connected with a macro base station, instead the femto basestation is connected via the Internet. Thus, the femto base station isnot capable of sharing information in real time. In addition, the femtobase station does not always know a pattern of a signal of the macroterminal, and does not know a radio resource used for transmission ofthe signal of the macro terminal. Thus, the femto base station incursdifficulty in recognizing a channel between the femto base station tothe macro terminal.

FIG. 3 illustrates a conventional example of a hierarchical cellcommunication system having difficulty sharing information in real time.

Referring to FIG. 3, multi-cell communication system 310 includes twomacro base stations, a macro base station A and a macro base station B.The macro base stations may be connected via a fiber optic cable or viaan X2 interface which is already well known in the art. In this example,the macro base station A and the macro base station B may have a slightdelay time when they share channel information in real time.

In the example hierarchical cell communication system 320, when themacro base station is connected with the femto base station via theInternet, the macro base station and the femto base station may includea lengthy delay time when sharing the channel information. Accordingly,the hierarchical cell communication system may have difficulty in usingthe cooperated beamforming and the like as discussed in 3GPPLTE_advanced standard or as discussed in an IEEE 802.16m standard.

FIG. 4 illustrates a conventional example of a hierarchical cellcommunication system including a plurality of femto base stations.

Referring to the example shown in FIG. 4, when the hierarchical cellcommunication system includes two or more femto base stations, there isa method by which a macro terminal measures interference channels (h₁,h₂). That is, the macro terminals may perform scanning of the femto basestation 1 and the femto base station 2, measure the interferencechannels (h₁, h₂), and report information about the interferencechannels to the macro base station.

However, the power of the macro terminal is limited, and a terminal mayexpend a great amount of power to separately measure interferencechannels from a plurality of femto base stations. Also, when the macroterminal reports the information about the interference channels to themacro base station, additional radio resources may be demanded.

FIG. 5 illustrates a process of an example hierarchical cellcommunication system.

Referring to FIG. 5, in operation 510, the femto base station transmitsa “Macro_Info_Request” to a macro base station via a wired or wirelessbackhaul, to obtain information related to a sounding signal of a macroterminal. In this example, the femto base station may receiveinformation from the macro base station. The sounding signal of themacro terminal may include a signal having a predetermined pattern or asignal having a predetermined bit, and is not limited to the “soundingsignal” of the 3GPP.

When the femto base station is authenticated, in operation 520, themacro base station transmits the information related to the soundingsignal of the macro terminal served by the macro base station. In thisexample, the information related to the sounding signal of the macroterminal is transmitted from the macro base station as message“Macro_Info.”

For example, the information related to the sounding signal of the macroterminal may include information about a pattern of the sounding signal,information about radio resource for the sounding signal, informationabout a transmission period of the sounding signal, and the like.Accordingly, the femto base station may recognize the pattern of thesounding signal transmitted from the macro terminal, the radio resourcesuch as a frequency resource and a time resource allocated for thesounding signal, a time when the sounding signal is transmitted, and thelike.

In operation 530, the femto base station transmits an acknowledgmentmessage “Macro_Info ACK” to the macro base station in response tosuccessful reception of the “Macro_Info.” message For example, the macrobase station may report to the macro terminal that the femto basestation is ready to receive the sounding signal from the macro terminalthrough the allocated radio resource.

Also, the macro base station may wirelessly determine that the femtobase station is disposed in a position capable of receiving the soundingsignal from the macro terminal. In operation 540, the macro base stationcommands the macro terminal to transmit the sounding signal, forexample, to transmit the sound signal based on at least one of adesignated pattern, an allocated radio resource, a designatedtransmission period, and the like.

In operation 550, the macro terminal transmits the sounding signal. Asmentioned above, the macro terminal may transmit the sound signal, forexample, based on the designated pattern, the allocated radio resource,the designated transmission period, and the like.

As described herein, the sounding signal is a signal transmitted by themacro terminal, and may be used by a femto base station for determining,for example, whether the macro terminal exists, an amount ofinterference, and/or for estimating a channel between the macro terminaland the femto base station. The sounding signal may include informationindicating a macro terminal, information about a radio resource for themacro terminal, information about a quality of service (QoS) of themacro terminal, and the like.

The pattern of the sounding signal may be determined differently basedon, for example, an allocation pattern of the radio resource for themacro terminal and/or the QoS of the macro terminal. The femto basestation may recognize the allocation pattern of the radio resource forthe macro terminal and/or the QoS of the macro terminal based on thepattern of the sounding signal. In some embodiments, the sounding signalmay be varied such that the femto base station may perform the describedfunction.

The femto base station may estimate a channel between the femto basestation and the macro terminal or may determine whether the macroterminal exists based on the sounding signal. For example, in a timedivision duplex system, the femto base station may estimate the channelfrom the macro terminal to the femto base station based on the soundingsignal, and may recognize the channel from the femto base station to themacro terminal. Also, the femto base station may determine that themacro terminal exists based on the sounding signal. The macro basestation may request feedback of information about the channel from thefemto base station or the macro terminal. The feedback information mayinclude, for example, information about a power of a signal of the femtobase station that is received from the macro terminal.

Accordingly, the macro base station and the femto base station may shareinformation about the channel between the femto base station and themacro terminal, and may perform an interference control, such as atransmission power control or a beamforming, based on the sharedinformation. Hereinafter, a long-term dynamic spectrum managementscheme, and a voluntary dynamic spectrum management scheme are describedas examples of the transmission power control, and a leakage-basedbeamforming is described as an example of the beamforming. Also, theshared information may be used for an inter-cell handover, and the like.

FIG. 6 illustrates an example of a change in transmission power of amacro base station and a femto base station as a macro terminal moves.

Referring to FIG. 6, in this example a macro terminal graduallyapproaches the femto base station in an environment where a macrocelluses the FFR scheme. As shown in graphs 610 and 620, the transmissionpower of the macro base station is not changed, and the transmissionpower of the femto base station in a frequency resource F3 for an outercell decreases.

For example, when the femto base station exists in the outer cell, thefemto base station may freely use a radio resource that is not used bythe macro terminal. Also, the femto base station may know a quality of aservice, such as an average signal to noise plus interference ratio(SINR), demanded by the macro terminal, and may restrictively use aradio resource used by the macro terminal based on a long-term channelgain, such as a path loss or shadowing, based on the assumption that thequality of the service is guaranteed.

When the femto base station determines that the macro terminal is notpresent, the femto base station may use all possible radio resource fromthe macro base station. Also, when the femto base station determinesthat the macro terminal is present, the femto base station mayeffectively use the radio resource such that the macro terminal mayperform communication without interference from the femto base station.

FIG. 7 illustrates a process of an example hierarchical cellcommunication system that applies a long-term dynamic spectrummanagement (DSM) scheme.

Referring to FIG. 7, in operation 710, a femto base station transmits a“Macro_Info_Request” to a macro base station via a backhaul to obtaininformation related to a sounding signal of a macro terminal.

In some embodiments, the femto base station and the macro base stationmay perform an authentication process. The macro base station may storeand update sounding signal information of the macro terminals stored bythe macro base station. When the femto base station is authenticated, inoperation 720, the macro base station transmits information related tothe sounding signal of the macro terminal to the femto base station. Inthis example, the information related to the sounding signal of themacro terminal is transmitted in a form of a message, “Macro_Info.”

In operation 730, the femto base station transmits an acknowledgmentmessage “Macro_Info ACK” to the macro base station in response tosuccessful reception of the “Macro_Info” message.

For example, the macro base station may report, to the macro terminal,that the femto base station is ready to receive a sounding signal fromthe macro terminal through an allocated radio resource.

In operation 740, the macro base station determines that the macroterminal is within signaling distance of the femto base station. Thatis, the macro base station determines that the femto base station iscapable of receiving the sounding signal from the macro terminal. Themacro base station commands the macro terminal to transmit the soundingsignal based on, for example, a designated pattern, allocated radioresource, a designated transmission period, and the like.

In operation 750, the macro terminal transmits the sounding signal basedon, for example, the designated pattern, the allocated radio resource,the designated transmission period, and the like.

In operation 760, the femto base station estimates or predicts a channelfrom the femto base station to the macro terminal.

The femto base station may estimate a long-term channel gain from themacro terminal to the femto base station, and thus, a long term channelgain from the femto base station to the macro terminal may be predicted.For example, in a time division duplex (TDD) system, when a channelreciprocity is established, the channel gain from the macro terminal tothe femto base station may be estimated as the channel gain from thefemto base station to the macro terminal.

For example, where the macro terminal frequently transmits the soundingsignal, the femto base station may estimate a long-term channel gain byaveraging measured channel gains. For example, where the macro terminaldoes not frequently transmit the sounding signal, the femto base stationmay estimate the long-term channel gain by adding an appropriate fadingmargin to the measured channel gains.

In operation 770, the femto base station transmits, to the macro basestation, the obtained long-term channel gain in a form of a message,“Channel Info Report.” The femto base station may use a wired backboneor a wireless backbone to transmit the long-term channel gain.

In operation 780, the macro base station calculates a transmission powerof each of femto base stations and a transmission power of eachfrequency resource (sub band) according to a transmission power control,based on the long-term channel gain. In operation 790, information aboutthe calculated transmission power is transmitted to each of the femtobase stations.

Unlike the diagram of FIG. 7, in some embodiments the transmission powermay be calculated by the femto base station, instead of the macro basestation. The femto base station may transmit the calculated transmissionpower to another femto base station and/or the macro base station.

The transmission power control may be based on the long-term channelgain and information that is generally available in the macro basestation. The transmission power control may be performed based on thefollowing algorithm.

$\Gamma_{m,i}^{o} = {{\frac{G_{m,i}p_{m,i}}{\sigma_{i}^{2} + {\sum\limits_{j = 1}^{F}{G_{f,{ij}}p_{f,j}}}}\frac{G_{m,i}p_{m,i}}{\sigma_{i}^{2} + {\sum\limits_{j = 1}^{F}{G_{f,{ij}}\left( {p_{f,j} + {\Delta \; p_{f,j}}} \right)}}}} \geq {\Gamma_{m,i}^{t}.}}$

In this example, p_(f,j) represents a current transmission power of afemto base station j, p_(m,i) represents a current transmission power ofa macro base station to a macro terminal I, Δp_(f,j) represents atransmission power change of femto base station j, G_(f,ij) represents achannel gain from a femto base station j to a macro terminal i, G_(m,i)represents a channel gain from a macro base station to a macro terminali, Γ_(m,i) ^(o) represents a long-term SINR of a macro terminal i,Γ_(m,i) ^(t) represents a long-term target SINR of a macro terminal i,and σ_(i) ² represents a background noise of macro terminal i.

Furthermore,

${A = \begin{bmatrix}\frac{G_{f,11}}{G_{m,1}p_{m,1}} & \cdots & \frac{G_{f,{1F}}}{G_{m,1}p_{m,1}} \\\vdots & \ddots & \vdots \\\frac{G_{f,{M\; 1}}}{G_{m,M}p_{m,M}} & \cdots & \frac{G_{f,{MF}}}{G_{m,M}p_{m,M}}\end{bmatrix}},{x = \begin{bmatrix}{\Delta \; p_{f,1}} & \cdots & {\Delta \; p_{f,F}}\end{bmatrix}^{\prime}}$ ${b_{t} = \begin{bmatrix}\frac{1}{\Gamma_{m,1}^{t}} & \cdots & \frac{1}{\Gamma_{m,M}^{t}}\end{bmatrix}^{\prime}},{b_{o} = \begin{bmatrix}\frac{1}{\Gamma_{m,1}^{o}} & \cdots & \frac{1}{\Gamma_{m,M}^{o}}\end{bmatrix}^{\prime}},{{Ax} = {b_{t} - b_{o}}},$

and G_(f,ij) is reported to the macro base station, after the femto basestation measures a sounding signal of the macro terminal.

In some embodiments, the number of femto base stations is greater thanthe number of macro terminals.

When the number of femto base stations is greater than the number ofmacro terminals, the algorithm considers minimizing the change of atransmission power ‘x’ of the femto base station, and considers thesatisfaction of a target long-term SINR as a constraint. For example,the algorithm may be expressed as given below.

min x s.t.  Ax = b_(t) − b_(o), andx_(o) = A^(H)(AA^(H))⁻¹(b_(t) − b_(o)).

In some embodiments, the number of femto base stations may be less thanor equal to the number of macro terminals.

When the number of femto base stations is less than or equal to thenumber of macro terminals, the algorithm considers minimizing thedeterioration of the efficiency of macro terminals. For example, thealgorithm may be expressed as given below.

min∥b _(t) −b _(o) −Ax∥ ², and x _(o)=(A ^(H) A)⁻¹ A ^(H)(b _(t) −b_(o)).

Based on a long-term DSM, the macro base station and the femto basestation exchange related information with each other to performtransmission power control. Also, according to a voluntary DSM, one ormore of the macro base station and the femto base station mayvoluntarily control its own transmission power.

FIG. 8 illustrates an example of a method of a femto base station in ahierarchical cell communication system that applies a voluntary DSMscheme.

Referring to FIG. 8, in operation 810, the femto base station performsmonitoring of a sounding signal of a macro terminal based on apredetermined period, for example, a transmission period.

In operation 820, the femto base station determines whether the soundingsignal of the macro terminal is detected.

When the sounding signal of the macro terminal is not detected, 810 isperformed again. If the sounding signal of the macro terminal isdetected, in operation 830, the femto base station determines whether apower (long-term gain) of the detected sounding signal is greater thanα. When the power of the detected sounding signal is greater than α, itis determined that the interference between the macro terminal and thefemto base station is above a preset threshold, and thus, a transmissionpower of the femto base station is reduced in operation 850. Conversely,when the power of the detected sounding signal is less than or equal toα, in operation 840, the femto base station determines whether the powerof the detected sounding signal is less than β. In this example, β is areal number that is less than α.

When the power of the detected sounding signal is less than β,interference between the femto base station and the macro terminal isdetermined to be below a preset threshold, and in operation 860, thefemto base station increases transmission power. In this example, anincrease or decrease of the transmission power indicates an increase ordecrease of a transmission power in a frequency band used by the macroterminal.

A voluntary DSM and a long-term DSM are based on a long-term channelgain. FIG. 9 illustrates an example in which interference control isbased on a short-term channel gain, such as a multi-path fading.

When the macro terminal transmits a sounding signal, the femto basestation may estimate a channel between the femto base station and themacro terminal. Also, when there are a plurality of femto base stations,each of the femto base stations may allocate its own radio resourcebased on the macro base station, in comparison to the example where themacro base station allocates radio resource to each of the femto basestations.

FIG. 9 illustrates an example of leakage-based beamforming

Referring to FIG. 9, the femto base stations 1-3, respectively, formbeams A, B, and C for femto terminals A, B, and C. In this example, thebeams A, B, and C are formed such that they do not generate interferencefor the macro terminal. In such a scheme, each of the femto basestations 1-3 may use a radio resource that is also used by the macroterminal, without causing interference.

Each of the femto base stations 1, 2, and 3 may generate a leakage-basedbeam forming vector based on the following Equations:

$\max \frac{{{H_{f,{kj}}w_{f,j}}}^{2}}{{M_{f,{kj}}\sigma^{2}} + {\sum\limits_{i = 1}^{M}{{H_{m,{ij}}w_{f,j}}}^{2}}}$s.t.  w_(f, j)² = 1, w_(f, j) ∈ C^(N_(f, j) × 1), and$w_{f,j}^{o} = {\max \mspace{14mu} {{eigenvector}\left( {\left( {{M_{f,{kj}}\sigma_{f,{kj}}^{2}I} + {{\overset{\sim}{H}}_{f,{kj}}^{*}{\overset{\sim}{H}}_{f,{kj}}}} \right)^{- 1}H_{f,{kj}}^{*}H_{f,{kj}}} \right)}}$

where {tilde over (H)}_(f,kj)=[H_(m,lj) . . . H_(m,Mj)]^(T)

In this example, N_(f,j) represents a number of transmission antennas ofa femto base station in a femtocell j, M_(f,kj) represents a number ofreception antennas of femto terminal k in a femtocell j, M_(m,i)represents a number of reception antennas of a macro terminal i,H_(f,kj) represents a channel matrix from a femto base station j to afemto terminal k, and having a dimension of M_(f,kj) by N_(f,j),H_(m,ij) represents a channel matrix from a femto base station j to amacro terminal i, and having a dimension of M_(m,i) by N_(f,j), w_(f,kj)represents a beamforming vector of a femto base station j for a femtoterminal k, and σ_(f,kj) represents a background noise power of a femtoterminal k in a femto cell j.

FIG. 10 illustrates an example of a method of femto base station in ahierarchical cell communication system that applies leakage-basedbeamforming.

Referring to FIG. 10, in operation 1010, the femto base station receivesa sounding signal of a macro terminal. The sounding signal may includechannel information. In operation 1020, the femto base station mayestimate a channel from the femto base station to the macro terminal,for example, a short-term channel gain. In operation 1020, the femtobase station may determine a channel between the femto base station andthe macro terminal, based on the information reported from the macroterminal.

In operation 1030, the femto base station selects at least one user(femto terminal) that is intended to be served, by using one or moreuser selection algorithms. For example, the femto base station may use aproportional fairness scheduling scheme.

In operation 1040, the femto bases station generates a leakage-basedbeamforming vector based on the above Equations.

FIG. 11 illustrates a process of an example hierarchical cellcommunication system that applies leakage-based beamforming.

Referring to FIG. 11, in operation 1110, the femto base stationtransmits a “Macro_Info_Request” to the macro base station via abackhaul to request information related to a sounding signal of themacro terminal.

In operation 1120, the macro base station transmits, to the femto basestation, information related to the sounding signal of the macroterminal that is served by the macro base station.

In operation 1130, the femto base station transmits an acknowledgmentmessage “Macro_Info ACK” to the macro base station in response tosuccessful reception of a message “Macro_Info.”

In operation 1140, the macro base station wirelessly determines that thefemto base station is capable of receiving a sounding signal from themacro terminal, and commands the macro terminal to transmit the soundingsignal. For example, the macro base station may command the macroterminal to transmit the sounding signal based on a designated pattern,allocated radio resource, a designated transmission period, and thelike.

In operation 1150, the macro terminal transmits the sounding signal ascommanded by the macro base station.

In operation 1160, the femto base station estimates a channel from thefemto base station to the macro terminal. For example, the femto basestation may estimate a channel including a short-term channel gain,based on the received sounding signal.

Unlike the diagram of FIG. 11, when a channel reciprocity is notestablished, information about a channel from the femto base station tothe macro terminal may be provided to the femto base station from themacro terminal. This is further described with reference to FIG. 12.

FIG. 12 illustrates another process of an example hierarchical cellcommunication system that applies leakage-based beamforming.

Referring to FIG. 12, operations 1210 through 1250 are performed in thesame manner as operations 1110 through 1150 of FIG. 11. Thus, adescription of these operations is omitted.

After operations 1210 through 1250 are performed, in operation 1260, thefemto base station recognizes whether the macro terminal exists, andalso recognizes an effect that the macro terminal has on the femto basestation.

In operation 1270, femto base station requests, from a macro basestation, information about a channel between the femto base station andthe macro terminal, and the macro base station responds to the requestin operation 1280. In operation 1290, the macro base station commandsthe macro terminal to feed back, to the femto base station, theinformation about the channel between the femto base station and themacro terminal. In operation 1291, the macro terminal feeds back, to thefemto base station, the information about the channel between the femtobase station and the macro terminal. In some embodiments, aninitialization operation for communication between the macro terminaland the femto base station is selectively operated.

In operation 1292, when the information about the channel from the femtobase station to the macro terminal is provided to the femto basestation, the femto base station generates an appropriate leakage-basedbeamforming vector.

According to example embodiments, the macro base station provides afemto base station with information about a pattern of a soundingsignal, information about radio resource for the sounding signal, andthe like, and thus, the femto base station is capable of receiving thesounding signal transmitted from a macro terminal. Accordingly, thefemto base station may estimate a channel between the macro terminal andthe femto base station or may recognize an existence of the macroterminal, and may reduce interference between of data sent from a macrobase station to the macro terminal.

The system described herein provides a solution to the problems causedby the interference from the femtocell to the macrocell, particularly,interference from the femto base station to the macro terminal, and mayshare channel information between the femtocell and the macrocell inreal time.

As a non-exhaustive illustration only, a terminal or a terminal devicedescribed herein may refer to mobile devices such as a cellular phone, apersonal digital assistant (PDA), a digital camera, a portable gameconsole, and an MP3 player, a portable/personal multimedia player (PMP),a handheld e-book, a portable lab-top personal computer (PC), a globalpositioning system (GPS) navigation, and devices such as a desktop PC, ahigh definition television (HDTV), an optical disc player, a setup box,and the like capable of wireless communication or network communicationconsistent with that disclosed herein.

A computing system or a computer may include a microprocessor that iselectrically connected with a bus, a user interface, and a memorycontroller. It may further include a flash memory device. The flashmemory device may store N-bit data via the memory controller. The N-bitdata is processed or will be processed by the microprocessor and N maybe 1 or an integer greater than 1. Where the computing system orcomputer is a mobile apparatus, a battery may be additionally providedto supply operation voltage of the computing system or computer.

It will be apparent to those of ordinary skill in the art that thecomputing system or computer may further include an application chipset,a camera image processor (CIS), a mobile Dynamic Random Access Memory(DRAM), and the like. The memory controller and the flash memory devicemay constitute a solid state drive/disk (SSD) that uses a non-volatilememory to store data.

The processes, functions, methods and/or software described above may berecorded, stored, or fixed in one or more computer-readable storagemedia that includes program instructions to be implemented by a computerto cause a processor to execute or perform the program instructions. Themedia may also include, alone or in combination with the programinstructions, data files, data structures, and the like. The media andprogram instructions may be those specially designed and constructed, orthey may be of the kind well-known and available to those having skillin the computer software arts. Examples of computer-readable mediainclude magnetic media, such as hard disks, floppy disks, and magnetictape; optical media such as CD-ROM disks and DVDs; magneto-opticalmedia, such as optical disks; and hardware devices that are speciallyconfigured to store and perform program instructions, such as read-onlymemory (ROM), random access memory (RAM), flash memory, and the like.Examples of program instructions include machine code, such as producedby a compiler, and files containing higher level code that may beexecuted by the computer using an interpreter. The described hardwaredevices may be configured to act as one or more software modules inorder to perform the operations and methods described above, or viceversa. In addition, a computer-readable storage medium may bedistributed among computer systems connected through a network andcomputer-readable codes or program instructions may be stored andexecuted in a decentralized manner.

A number of examples have been described above. Nevertheless, it shouldbe understood that various modifications may be made. For example,suitable results may be achieved if the described techniques areperformed in a different order and/or if components in a describedsystem, architecture, device, or circuit are combined in a differentmanner and/or replaced or supplemented by other components or theirequivalents. Accordingly, other implementations are within the scope ofthe following claims.

1. A transmission power control method of a base station, the methodcomprising: receiving sounding signal information of a macro terminalfrom a macro base station; detecting a sounding signal transmitted bythe macro terminal based on the received sounding signal information;estimating a channel between the macro terminal and the base stationbased on the detected sounding signal; and processing information aboutthe estimated channel to control a transmission power of the macro basestation and the base station.
 2. The method of claim 1, furthercomprising requesting the sounding signal information from the macrobase station.
 3. The method of claim 1, further comprising transmitting,to the macro base station, an acknowledgment message indicating that thesounding signal information was received.
 4. The method of claim 1,wherein the detected sounding signal indicates at least one ofinformation identifying the macro terminal, information about a radioresource used by the macro terminal, and information about a quality ofservice (QoS) of the macro terminal.
 5. The method of claim 1, whereinthe sounding signal information of the macro terminal comprises at leastone of information about a pattern of the sound signal, informationabout radio resource for the sounding signal, and information about atransmission period of the sounding signal.
 6. The method of claim 1,wherein the processing of the information about the estimated channelcomprises transmitting information about the estimated channel to themacro base station to control the transmission power of the macro basestation and the base station.
 7. The method of claim 1, wherein theprocessing of the information about the estimated channel comprisesdetermining the transmission power of the base station based on theinformation about the estimated channel.
 8. The method of claim 6,further comprising receiving information about the controlledtransmission power of the base station from the macro base station.
 9. Atransmission power control method of a base station, the methodcomprising: receiving sounding signal information of a macro terminalfrom a macro base station; detecting a sounding signal transmitted bythe macro terminal, based on the received sounding signal information ofthe macro terminal; and adjusting the transmission power of the basestation based on the power of the detected sounding signal.
 10. Themethod of claim 9, wherein the adjusting comprises: comparing the powerof the detected sounding signal with at least one comparison value; andincreasing or decreasing the transmission power based on a result of thecomparison.
 11. A transmission power control method of a base station,the method comprising: receiving sounding signal information of a macroterminal from a macro base station; detecting a sounding signaltransmitted by the macro terminal based on the received sounding signalinformation of the macro terminal; and performing one of: estimating achannel between the macro terminal the base station based on the soundsignal and generating a beamforming vector for a femto terminal based onthe estimated channel; or receiving information about a channel betweenthe macro terminal and the base station from the macro terminal inresponse to the detection of the sounding signal, and generating thebeamforming vector for the femto terminal based on the receivedinformation about the channel.
 12. The method of claim 11, furthercomprising requesting the information about the channel between themacro terminal and the base station from the macro terminal, in responseto the detection of the sounding signal.
 13. A transmission powercontrol method of a macro terminal, the method comprising: receiving arequest, from a macro base station, to transmit a sounding signal; andtransmitting the sounding signal to a base station, wherein the soundingsignal indicates at least one of information identifying the macroterminal, information about radio resource for the macro terminal, andinformation about a QoS of the macro terminal, and sounding signalinformation is provided in advance to the base station from the macrobase station, the sounding signal information including at least one ofinformation about a pattern of the sounding signal, information aboutradio resource for the sounding signal, and information about atransmission period of the sounding signal.
 14. A computer-readablestorage medium storing a program to control transmission power of a basestation, the computer-readable storage medium comprising instructions tocause a computer to: receive sounding signal information of a macroterminal from a macro base station; detect a sounding signal transmittedby the macro terminal based on the sounding signal information of themacro terminal; estimate a channel between the macro terminal and thebase station based on the detected sounding signal; and processinformation about the estimated channel to control the transmissionpower of the macro base station and the base station.
 15. Acomputer-readable storage medium storing a program to controltransmission power of a macro terminal, the computer-readable storagemedium comprising instructions to cause a computer to: receive arequest, from a macro base station, to transmit a sounding signal; andtransmit the sounding signal to a base station, wherein the soundingsignal indicates at least one of information identifying the macroterminal, information about radio resource for the macro terminal, andinformation about a QoS of the macro terminal, and sounding signalinformation is provided in advance from the macro base station to thebase station, the sounding signal information including at least one ofinformation about a pattern of the sounding signal, information aboutradio resource for the sounding signal, and information about atransmission period of the sounding signal.